For certain types of reflective optical systems it is important that the component reflecting surfaces have low total optical scatter. For the purposes of this application, the total optical scatter is defined as the ratio of the energy diffusely scattered by a reflecting surface into a 2.pi. solid angle (a hemisphere) from a beam of light incident on the surface to the energy of the incident beam.
One type of system requiring such low scatter mirrors is the ring laser gyro. In a ring laser gyro two laser beams are generated in opposite directions of propagation about a closed loop path formed by three or more mirrors. The path encloses the axis of rotation about which an angular rotation rate is to be sensed. Rotation of the apparatus about this axis causes a frequency difference between the two beams which provides a readout of rotation rate. However, difficulty arises at low rotational rates because the scattering of the beams from the mirrors causes the two beams to tend to oscillate at only one frequency or to "lock-in." Lock-in makes it impossible to measure rotational rates because the frequency difference disappears even though the rotation rate of the gyro is not zero.
As is well known the lower the mirror scattering, the lower is the lock-in frequency and hence the lower the minimum measurable rotation rate. Ring laser gyros therefore require mirrors having extremely low scattering in order to adequately measure low rotation rates. As an example, a satisfactory mirror must typically scatter no more than a few parts in ten thousand of the incident laser beam. Fabrication of such mirrors requires the deposition on a mirror substrate of a reflective coating comprising a stack of dielectric layers specifically designed for low scatter. Such a reflective coating is described, for example, in U.S. Pat. No. 4,142,958 issued to D. Wei and A. Louderback on Mar. 6, 1979 and assigned to the assignee of the present invention.
Efficient fabrication of these mirrors requires a device called a scatterometer for measuring the amount of light each mirror scatters prior to its installation in a laser gyro. A problem which arises is that conventional scatterometers lack the capability to easily and accurately measure the low levels of light involved. In one such conventional instrument, an integrating sphere scatterometer, the light scattered from the mirror sample being measured is directed into the interior of a sphere having a coating which diffusely scatters light falling upon it. Thus scattered, the light uniformly illuminates the interior of the sphere. A photosenser views the interior surface through a small window in the wall of the sphere and thereby produces an output signal proportional to the total scattering of the mirror. One of the drawbacks of the integrating sphere scatterometer is the low measurement sensitivity resulting from the fact that only a small fraction of the total light scattered is intercepted by the photosensor. Hence, low level scattering is difficult to measure.
Another type of prior art scatterometer measures scattering over a range of small solid angles. By adding up the scattering from each increment of solid angle, a total scatter is obtained. However, equipment to accomplish this is expensive and the measurement process is laborious.